1. Field of the Invention
This invention relates to polyurethane one-component baking enamels containing solvents, consisting of polyisocyanates exhibiting isocyanurate or urethane groups totally or partially blocked with blocking agents known in the art and having a base of 2-methyl-1,5-diisocyanatopentane (MPDI) or a mixture of MPDI and 2-ethyl-1,4-diisocyanatobutane (EBDI) and polyesters containing hydroxyl with a base of mainly aromatic dicarboxylic acids, diols and triols.
2. Description of the Prior Art
For the production of mixtures of polyisocyanates and polyoxy compounds that are stable at the usual temperatures, as is known, polyisocyanates are used whose reactive groups are closed by reaction with monofunctional compounds and generally are designated as masked or blocked polyisocyanates. Products of this type are described, for example, in Annalen, Volume 562 (1949) pp. 205-229.
At higher temperatures, the blocked polyisocyanates break down into their initial components, whereby the isocyanate groups, which are released, in turn can react with the polyoxy compounds that are present in the reaction mixture.
If these mixtures of blocked polyisocyanates and polyoxy compounds are to be applied for coating objects by the spraying or curtain coating process and for coating of sheet metal by roll coating, especially by the coil coating process, both the cross-linking agent and the resin must be soluble in conventional varnish solvents and be stable in storage.
With only few exceptions, aliphatic polyisocyanates blocked with .epsilon.-caprolactam or acetoxime, are incompatible with conventional varnish solvents. They are, therefore, eliminated as initial components for production of PUR one-component baking enamels preferably for coating metal objects with highly deformable coatings.
Production of hexamethylenediisocyanate (HDI) blocked with .epsilon.-caprolactam or acetoxime in the usual varnish solvents is thwarted by incompatibility. These products precipitate during or shortly after the reaction. Even partial blocking of HDI and subsequent reaction with the polyoxy compounds was able to give the varnish solution only a short-lived stability in storage. An improvement, of course not yet satisfactory for practice, can be achieved by use of blocked HDI isocyanurate. The compatibility with polyhydroxyl compounds generally used in PUR varnishes is only partial and solubility is limited especially in nonpolar solvents such as mineral spirits. A further drawback of those HDI isocyanurates is their limited thermal and oxidation stability, which is seen in practice in overbaking, e.g., caused when the conveyor is stopped during the hardening process. The results are quality losses such as, for example, discoloration of the enamel films.
Therefore, a need clearly continues to exist for polyisocyanates which, although totally or partially blocked, are compatible with conventional varnish solvents and which have excellent storage stability in such varnish solvents.